I believe it would be by Upper Hand, since its 4x supereffective and Kingambit is known for Sucker Punch sweeping, but if someone has a better combo, I'm here to listen
First, Kingambit has no significant niches in Reg G, so it should be easy to counter if you use standard Reg G Pokemon like Incineroar, rapid strike Urshifu, Iron Hands, or hearthflame Ogerpon. source
In AG, Kingambit rarely uses coverage, so it's reasonably easy to check with a Pokemon that has a super effective move and isn't weak to dark or steel, like Ho-Oh, Gliscor, ground Arceus, or therian Landorus. There are also several reliable ways to defensively counter Kingambit. Water Arceus can burn it, Toxapex can stop it from setting up, and Dondozo can ignore its stat changes. Skeledirge can burn it and ignore its stat changes, but of course Skeledirge is weak to dark if it doesn't Tera. source source
"Countering" Kingambit can mean a couple things, but I'll answer the best I can.
Countering in terms of taking it down quickly: Having a Fighting type such as Hariyama or Great Tusk is easily the best way to take down Kingambit since it's a Dark/Steel type, and make sure to give the Fighting type that you have some coverage in case the Kingambit Terastalizes. Make sure to be careful if the Fighting type you use is weak to one of Kingambit's types, such as Iron Valiant's Steel weakness.
Countering in terms of walling it: Make sure you have a Pokemon that resist's Kingambit's types(Fighting, Fairy and Dark types resist the Dark type and Fire, Water, Electric and Steel types resist the Steel type). Pokemon that resist both types such as Tinkaton(Steel/Fairy) or Rotom-Heat(Fire/Electric) will work great. If you don't have a Pokemon that resists either type, you can have a physically defensive Pokemon instead like Probopass or maybe a Terastalizes Garganacl.
Hope this helps!
